The present invention relates to a motor-vehicle radar system.
Motor-vehicle radar systems are used within the framework of an automatic cruise control system of a vehicle for detecting vehicles driving ahead. A system of this type is also known as Adaptive Cruise Control (ACC). To influence the electromagnetic waves used and occasionally to protect the radar system from weather influences, as well, usually a member is situated in the beam path of the electromagnetic waves. This member is often a component of a housing which surrounds such a motor-vehicle radar system.
The German Published Patent Application No. 197 24 320 describes a method for producing a heatable antenna lens. A heatable antenna lens is described made of a dielectric member which possesses an arrangement of electrical conductor tracks therein. The arrangement of electrically conductive tracks is situated as near as possible to the outer surface of the lens to be heated, a reduction of heating power thereby resulting by introducing the energy close underneath the surface to be heated. Also resulting from this is an accelerated heat-up characteristic. It is furthermore described that easy adaptability of the heating power can be achieved by using wires having a desired resistance characteristic. For example, it can be a resistance wire.
The German Published Patent Application No. 196 26 344 describes a lens for focusing millimetric waves, particularly for distance sensors for motor vehicles, which are made at least partially of a ceramic material or a plastic, e.g. polypropylene or polycarbonate, filled with ceramic material. For example, aluminum oxide with magnesium-oxide and potassium-oxide constituents is provided as ceramic material. Lenses made purely of a ceramic material can be produced according to German Published Patent Application No. 196 26 344 by pressing, tempering or sintering, and lenses made of plastics filled with ceramic material can be produced in the injection-molding process. One specific embodiment of German Published Patent Application No. 196 26 344 provides for applying a protective coating on the surface of the lens which protects the outwardly-directed lens surface at least from weather influences. Hexamethyl disiloxane is applicable for the protective coating in particular for plastic lenses filled with ceramic material. A further form of German Published Patent Application No. 196 26 344 provides for a convex-plane lens, the convex or the plane surface being alternatively subject to the weather influences in the installed state. To reduce reflection losses, German Published Patent Application No. 196 26 344 allows for providing at least one surface of the lens with grooves which can be filled with a dielectric material in order to lend a plane surface to the lens. The dielectric material can have an inhomogeneous distribution of the dielectric constant in the direction of the optical axis of the lens and/or transverse thereto.
The German Patent No. 197 41 081 describes a method for producing an antenna lens. The method provides that a rigid shell, predetermining the shape of the lens and forming the anti-reflection layer, is produced, and that the cavity of the shell is filled in with a liquefied, powdery or pasty material forming the lens core. The shell can be made of a plastic (for example, plexiglass, polycarbonate or similar materials), while, for example, plastics and mixtures of plastic with ceramic materials (e.g. polyethylene, polypropylene, Al2O3, TiO2 or CaZOx) are provided as material for the lens core. According to German Patent No. 197 41 081, the surface of the shell can be designed both to be smooth, and can have any structure at all, for example, a Fresnel structure. So that the shell forms an ideal anti-reflection layer for the lens, German Patent No. 197 41 081 C1 proposes that the shell be manufactured from a plastic whose dielectric constant corresponds to the square root of the dielectric constant of the actual lens material (the filling). Furthermore, it can be inferred from German Patent No. 197 41 081 that, by very high dielectric constants (xcex5 greater than 9) of the dielectric lens material, compact designs can be implemented having short focal lengths.
From the German Patent No. 196 44 164, a motor-vehicle radar system is known having at least one send/receive element for sending and/or receiving electromagnetic waves, a lens-shaped dielectric member being arranged in the beam path of the at least one send/receive element to focus or scatter the electromagnetic waves. The lens-shaped dielectric member, which additionally protects the send/receive element from weather influences, has an arrangement of electrically conductive tracks whose breadth is tenth wave at the most, and whose distances from one another are at least a quarter wave, Lambda designating the free-space wavelength of the electromagnetic waves. The electrically conductive tracks are arranged predominately in a direction perpendicular to the polarization direction of the electromagnetic waves. Depending on the desired use, the arrangement of electrically conductive tracks can be disposed on the inner side of the dielectric member, i.e. the side facing the send/receive elements, the outer side, or even within the dielectric member. If a heating current flows through the electrically conductive arrangement, the dielectric member can thereby be freed from coatings such as ice, snow or slush. In the same way, the dielectric member can be dried or kept dry with the aid of a heating current. It is also disclosed that the electrically conductive arrangement may be subdivided into at least two portions separate from each other. When working with this configuration, if the arrangement of electrically conductive tracks is on the outer side of the dielectric member, it is possible to infer a so-called loss angle tan xcex4 of the coating material on the basis of the measurement of the capacitance between the two separate portions of the arrangement. In other words, a soiling of the dielectric member can be determined. In dependence on this determined soiling, i.e. a determined dirt coating, a heating current can be switched on which flows through the electrically conductive arrangement. On the other hand, due to the division into at least two regions, the heating power can be varied, for example, to quickly heat up an ice-covered lens using a high heating power, and subsequently keeping the lens free using a reduced heating power. It is further known from German Patent No. 196 44 164 that when working with a lens member made of ceramic, the electrical conductor tracks are applied using known thick-film technology, while for members made of plastic, likewise known, cost-effective methods can be used for imprinting the electrical conductor tracks. It can be inferred from one exemplary embodiment in German Patent No. 196 44 164 that a lens can be provided, composed of two partial lenses, which is made of two plane-convex partial lenses of different materials. In this case, the electrical conductor tracks can be arranged in one plane between the plane surfaces of the two partial lenses.
German Published Patent Application No. 197 24 320, German Published Patent Application No. 196 26 344, German Patent No. 197 41 081 and German Patent No. 196 44 164 describe various forms of antenna lenses which can be used in a motor-vehicle radar system. German Published Patent Application No. 197 24 320 and German Patent No. 196 44 164 each include an arrangement of electrical conductor tracks which can be used, inter alia, to free the side of the antenna lens exposed to the influences of weather from coatings such as snow, ice or slush. Both patents describe the possibility of regulating the heating power, either in that the heating power can be adapted by wires having a desired resistance characteristic, or in that the electrically conductive arrangement is subdivided into at least two portions separate from one another.
Given a motor-vehicle radar system having at least one housing, having at least one send/receive element for sending and/or receiving electromagnetic waves, having at least one first dielectric member in the beam path of the electromagnetic waves, having at least one arrangement of electrical conductor tracks which are in direct contact with the first dielectric member, it being possible to supply an electric power to the electrical conductor tracks and the electrical conductor tracks preferably being used to heat the first dielectric member, the motor-vehicle radar system of the present invention is further developed compared to the related art in that the electrical conductor tracks are made of a material having a pronounced, positive temperature coefficient (PTC characteristic). The result of this use, according to the present invention, of a material with a pronounced, positive temperature coefficient is that, with rising temperature, the electrical resistance of the electrical conductor tracks increases very sharply. A type of self-regulating behavior thereby sets in, in that as the temperature rises, the heating power is reduced because of the increasing resistance. Assuming a constant motor-vehicle electrical-system voltage, this means that the heating power is inversely proportional to the heating power. Suitable dimensioning of the electrical conductor tracks and the selection of the appropriate material with pronounced PTC characteristic eliminates the need for a complicated and cost-creating open-loop or closed-loop control of the electrical heating power, since this function is ensured by the self-regulating effect of the electrical conductor tracks.
Advantageously, the distance of the electrical conductor tracks to one another corresponds essentially to an odd multiple of a quarter of the free-space wavelength of the electromagnetic waves used. This design results in a minimal influencing of the electromagnetic waves.
The preferred further development of the motor-vehicle radar system according to the present invention provides that the first dielectric member is a lens for focusing or scattering the electromagnetic waves.
An advantageous further refinement is that the first dielectric member is made of a ceramic material, and that the electrical conductor tracks are produced from an electrically conductive printing paste which is applied on the dielectric-member surface facing the send/receive elements. Due to this constructive design of the present invention, a type of xe2x80x9cceramic heating stonexe2x80x9d having self-regulating effect is produced. In this context, it is advantageous if the electrical conductor tracks form a level arrangement, since this is associated with the lowest costs with respect to producing the electrical conductor tracks.
Another further development provides that a covering layer, made of a thermoplastic material, is applied on the first dielectric member at least on the side facing away from the send/receive elements, the covering layer enclosing the first dielectric member in such a way that the covering layer represents a seal with respect to the housing. In this case, the covering layer is dimensioned such that the dielectric constant of the covering layer corresponds approximately to the square root of the dielectric constant of the first dielectric member, and that the layer thickness of the covering layer corresponds to an odd multiple of a quarter of the free-space wavelength of the electromagnetic waves used. This arrangement and dimensioning of the covering layer according to the present invention permits the covering layer to be used as a reflection-damping adaptation layer which protects the motor-vehicle radar system from weather influences.
A further refinement of the motor-vehicle radar system according to the present invention provides that at least one second dielectric member is arranged in the beam path of the electromagnetic waves, the second dielectric member being a lens for focusing or scattering the electromagnetic waves, and the second dielectric member being arranged between the send/receive elements and the first dielectric member. This design approach offers the advantage that the dielectric lens for focusing the electromagnetic waves can be produced independently of the first dielectric member. In this case, the preferred layout is such that the second dielectric member is not in direct contact with the first dielectric member.